support more ssl versions

[guanqun]

I see that tlsv1.1 and tlsv1.2 are commented out in the code, see the following line:

curl-rust/src/easy/handler.rs

Line 484 in cf49823

// Tlsv12 = curl_sys::CURL_SSLVERSION_TLSv1_2 as isize,

any specific reason? I'd like to extend this SslVersion to support more versions. If you're OK, I can help send a simple PR.

[alexcrichton]

I think that was jut an artifact of when they were first added, should be fine to uncomment now!

[guanqun]

Thanks for the clarification, I've now created a PR to uncomment these enums and added a Tlsv13 too. Please help review. Thanks.

[guanqun]

Thanks for the merge, can we bump a version for this small feature?

[alexcrichton]

Sure! I noticed though that CI is failing on master, would you be willing to take a look at that? (I think it's caused by this PR)

[guanqun]

Sure! I checked the error log:

cargo:warning=             static const CURLoption __test_const_CURL_SSLVERSION_TLSv1_3_val = CURL_SSLVERSION_TLSv1_3;
cargo:warning=                                                                                ^
cargo:warning=cc1: error: unrecognized command line option "-Wno-address-of-packed-member" [-Werror]
cargo:warning=cc1: all warnings being treated as errors
exit code: 1

So it's due to the TLSv1_3 being not defined. I'm guessing that on that docker image, the curl header/library is too old.

Notice that Dockerfile-linux64-curl uses ubuntu 14.04 while Dockerfile-linux64 uses ubuntu 16.04 and 16.04's version runs well for systest, see the below log:
https://travis-ci.com/alexcrichton/curl-rust/jobs/179317628

So I'd suggest upgrading Dockerfile-linux64-curl to use Ubuntu 16.04. If you agree this is a good move, I can create a simple PR.

[alexcrichton]

That makes sense! We've got a number of other cases of constants like that, so I think this'd just need to be added to that list?